Early onset cardiomyopathy in females with Danon disease

Altered Splicing of LAMP2 in a Multigenerational Family from Latvia Affected by Danon Disease

Background and Objectives: Danon disease is a multisystemic disorder associated with variants in the LAMP2 gene, mainly affecting the cardiac muscle. Here, we report a multigenerational family from Latvia with two male patients, hemizygous for a novel splice-affecting variant c.928+3A>G. Affected patients exhibit a cardiac phenotype, moderate mental disability, and mild retinal changes. Materials and Methods: Both patients underwent either exome or hypertrophic cardiomyopathy gene panel next-generation sequencing. The pathogenic variant effect was determined using reverse transcription, Sanger sequencing, and high-resolution electrophoresis. Results: Evaluation of the splicing process revealed that approximately 80% of the transcripts exhibited a lack of the entire exon 7. This alteration was predicted to cause a shift of the reading frame, consequently introducing a premature stop codon downstream in the sequence. Conclusions: Based on our data, we propose that c.928+3A>G is a pathogenic variant associated with Danon disease.

International Consensus on Differential Diagnosis and Management of Patients With Danon Disease: JACC State-of-the-Art Review

Danon disease is a rare X-linked autophagic vacuolar cardioskeletal myopathy associated with severe heart failure that can be accompanied with extracardiac neurologic, skeletal, and ophthalmologic manifestations. It is caused by loss of function variants in the LAMP2 gene and is among the most severe and penetrant of the genetic cardiomyopathies. Most patients with Danon disease will experience symptomatic heart failure. Male individuals generally present earlier than women and die of either heart failure or arrhythmia or receive a heart transplant by the third decade of life. Herein, the authors review the differential diagnosis of Danon disease, diagnostic criteria, natural history, management recommendations, and recent advances in treatment of this increasingly recognized and extremely morbid cardiomyopathy.

Identification and functional analysis of a novel de novo missense mutation located in the initiation codon of LAMP2 associated with early onset female Danon disease

BACKGROUND

Danon disease is characterized by the failure of lysosomal biogenesis, maturation, and function due to a deficiency of lysosomal membrane structural protein (LAMP2).

METHODS

The current report describes a female patient with a sudden syncope and hypertrophic cardiomyopathy phenotype. We identified the pathogenic mutations in patients by whole-exon sequencing, followed by a series of molecular biology and genetic approaches to identify and functional analysis of the mutations.

RESULTS

Suggestive findings by cardiac magnetic resonance (CMR), electrocardiogram (ECG), and laboratory examination suggested Danon disease which was confirmed by genetic testing. The patient carried a novel de novo mutation, LAMP2 c.2T>C located at the initiation codon. The quantitative polymerase chain reaction (qPCR) and Western blot (WB) analysis of peripheral blood leukocytes from the patients revealed evidence of LAMP2 haploinsufficiency. Labeling of the new initiation codon predicted by the software with green fluorescent protein followed by fluorescence microscopy and Western blotting showed that the first ATG downstream from the original initiation codon became the new translational initiation codon. The three-dimensional structure of the mutated protein predicted by alphafold2 revealed that it consisted of only six amino acids and failed to form a functional polypeptide or protein. Overexpression of the mutated LAMP2 c.2T>C showed a loss of function of the protein, as assessed by the dual-fluorescence autophagy indicator system. The mutation was confirmed to be null, AR experiments and sequencing results confirmed that 28% of the mutant X chromosome remained active.

CONCLUSION

We propose possible mechanisms of mutations associated with haploinsufficiency of LAMP2: (1) The inactivation X chromosome carrying the mutation was not significantly skewed. However, it decreased in the mRNA level and the expression ratio of the mutant transcripts; (2) The identified mutation is null, and the active mutant transcript fails to translate into the normal LAMP2 proteins. The presence of haploinsufficiency in LAMP2 and the X chromosome inactivation pattern were crucial factors contributing to the early onset of Danon disease in this female patient.

Clinical features of Danon disease and insights gained from LAMP-2 deficiency models

Danon disease (DD) is an X-linked multisystem disorder with clinical features characterized by the triad of hypertrophic cardiomyopathy, skeletal muscle weakness, and mental retardation. Cardiac involvement can be fatal in the absence of an effective treatment option such as heart transplantation. Molecular studies have proved that LAMP-2 protein deficiency, mainly LAMP-2B isoform, resulting from LAMP2 gene mutation, is the culprit for DD. Autophagy impairment due to LAMP-2 deficiency mediated the accumulation of abnormal autophagic vacuoles in cells. While it is not ideal for mimicking DD phenotypes in humans, the emergence of LAMP-2-deficient animal models and induced pluripotent stem cells from DD patients provided powerful tools for exploring DD mechanism. In both in vitro and in vivo studies, much evidence has demonstrated that mitochondria dysfunction and fragmentation can result in DD pathology. Fundamental research contributes to the therapeutic transformation. By targeting the molecular core, several potential therapies have demonstrated promising results in partial phenotypes improvement. Among them, gene therapies anticipate inaugurate a class of symptom control and prevention drugs as their in vivo effects are promising, and one clinical trial is currently underway.

Danon disease: a case report and literature review

BACKGROUND

Danon disease (DD) is a rare x-linked dominant multisystemic disorder with a clinical triad of severe cardiomyopathy, skeletal myopathy, and mental retardation. It is caused by a defect in the lysosomal-associated membrane protein-2 (LAMP2) gene, which leads to the formation of autophagic vacuoles containing glycogen granule deposits in skeletal and cardiac muscle fibers. So far, more than 50 different mutations in LAMP2 have been identified.

CASE PRESENTATION

Here, we report an 18-year-old male patient who was hospitalized for heart failure. Biopsy of the left lateral femoral muscle revealed scattered autophagic vacuoles in the muscle fibers with increased glycogen. Next generation sequencing (NGS) was used to detect gene mutations of the proband sample and a novel frameshift mutation (c.1052delG) has been identified in exon 8 of LAMP2, which leads to truncation of the protein.

CONCLUSION

We found a novel frameshift mutation, a hemizygous mutation (c.1052delG) in exon 8 of LAMP2, identified as presenting the hypertrophic cardiomyopathy (HCM) phenotype. Genetic analysis is the gold standard for the diagnosis of DD and is essential to determine appropriate treatment strategies and to confirm the genetic risk of family members.

Dosage Compensation in Females with X-Linked Metabolic Disorders

Through the use of new genomic and metabolomic technologies, our comprehension of the molecular and biochemical etiologies of genetic disorders is rapidly expanding, and so are insights into their varying phenotypes. Dosage compensation (lyonization) is an epigenetic mechanism that balances the expression of genes on heteromorphic sex chromosomes. Many studies in the literature have suggested a profound influence of this phenomenon on the manifestation of X-linked disorders in females. In this review, we summarize the clinical and genetic findings in female heterozygotic carriers of a pathogenic variant in one of ten selected X-linked genes whose defects result in metabolic disorders.

Clinical and molecular characterization of seven patients with Danon disease

Danon disease is an X-linked glycogen storage disease characterized by skeletal myopathy, cardiomyopathy and intellectual impairment. It is caused by a loss-of-function mutation in the lysosome-associated membrane protein-2 (LAMP2) gene. In the present study, exon and boarding intron analysis of 96 cardio disease-associated genes was performed in 770 patients with hypertrophic cardiomyopathy (HCM) using second-generation sequencing. Next, the identified mutations were confirmed in family members of the patients and 300 healthy controls. Detailed clinical, electrocardiographic (ECG) and echocardiographic findings were recorded. A pathogenic mutation in LAMP2 was identified in 7 patients who phenotypically presented with HCM. A total of four patients had a fragmented QRS complex (fQRS) on surface ECG. In addition, two patients presented with ventricular preexcitation with a short PR interval. Compared with the patients with protein kinase AMP-activated non-catalytic subunit γ2 syndrome and Fabry disease, the 7 patients with Danon disease presented at an earlier age, had a smaller left atrial size, a thinner maximal left ventricular wall thickness and a lower probability of pacemaker implantation. Compared with 12 sex- and age-matched patients with sarcomere-protein mutations, the 4 patients with Danon disease had a lower left ventricular outflow tract gradient and worse diastolic function. The present study provided a comprehensive comparison of different pathologies presenting with HCM and reported on features of early-onset Danon disease, including the characteristic preexcitation and fQRS on ECG. This may provide valuable information that may be utilized for the early diagnosis and treatment of patients with Danon disease. The present study was registered as a clinical trial with ClinicalTrials.gov (Sep. 2, 2016; registry no. NCT02888132).

Case Report: Identification of Mutations in LAMP2 in Two Chinese Infants With Danon Disease

Danon disease (DD) is a monogenic lysosomal storage disorder characterized by cardiomyopathy, skeletal myopathy, and variable degrees of intellectual disability. It is caused by a deficiency of lysosomal-associated membrane protein 2 (LAMP2). Two unrelated boys who presented with severe hypertrophic cardiomyopathy and elevated levels of liver enzymes, and were diagnosed with Danon disease at a very young age, were investigated. One boy was diagnosed at 4 months old and died soon after; his mother also died of hypertrophic cardiomyopathy shortly after his birth. Another developed hypertrophic cardiomyopathy at 3 months old but reported no significant cardiovascular symptoms during more than 5 years follow-up. Genetic screening found compound variants of LAMP2 and MYH7 in both of them. This report highlights the clinical heterogeneity in DD. The timely identification of LAMP2 mutation plays a critical role in their treatment and family counseling.

Danon disease: Two patients with atrial fibrillation in a single family and review of the literature

The present study reports on a family with two members affected by Danon disease but having different phenotypes. The clinical manifestations of Danon disease include cardiomyopathy, skeletal myopathy and different degrees of intellectual disability that varies greatly among patients. The present case study reports on two siblings, an older sister and a younger brother, with Danon disease from an affected pedigree, presenting with distinctly different phenotypes. The sister was diagnosed with dilated cardiomyopathy at the age of 26 years with an unfavorable outcome, while her younger brother presented with hypertrophic cardiomyopathy in a relatively stable state. The two probands shared the same mutation, c.974delTinsAA in exon 8, in the lysosomal-associated membrane protein-2 gene. Of note, the two patients had a pre-excitation pattern in the electrocardiogram on initial presentation and later developed atrial fibrillation (AF), which markedly aggravated heart failure. To the best of our knowledge, AF has not been widely reported in patients with Danon disease. The development of AF may have a prognostic value under these circumstances.

A novel LAMP2 p.G93R mutation associated with mild Danon disease presenting with familial hypertrophic cardiomyopathy

BACKGROUND

Danon disease (DD) is an X-linked dominant multisystem disorder that is associated with cardiomyopathy, skeletal myopathy, and varying degrees of intellectual disability. It results from mutations in the lysosome-associated membrane protein 2 (LAMP2) gene.

METHODS

Herein, a proband with a mild DD case presenting with a familial hypertrophic cardiomyopathy (HCM) phenotype and additional family members were evaluated. Exome sequencing and Sanger sequencing were performed to explore the genetic basis of DD in the proband. Segregation, in silico, and functional analyses were carried out to explore potential pathogenicity in the candidate mutation.

RESULTS

Exome sequencing and Sanger sequencing identified one novel missense mutation (p.G93R) in the LAMP2 gene in the proband, and this mutation was also identified in three other family members. In silico analysis of LAMP2 predicted that the mutation causes a conformational change and subsequent protein destabilization. Furthermore, functional examination showed that mutation carriers have a significant reduction in LAMP2 expression, which supports that the mutation is pathogenic. Moreover, skewed X chromosome inactivation (XCI) was identified in one female mutation carrier, thus suggesting that skewed XCI may be the reason why this individual escaped the pathogenic influence of the mutation.

CONCLUSION

These findings will aid in diagnosing DD patients carrying this LAMP2 mutation that presents with a HCM phenotype. Furthermore, this study illustrates the importance of utilizing a molecular diagnostic approach in HCM patients and is the first study to report a LAMP2 p.G93R mutation associated with mild DD and identify that XCI serves a protective role in DD etiology.

Danon disease presenting with early onset of hypertrophic cardiomyopathy and peripheral pigmentary retinal dystrophy in a female with a de novo novel mosaic mutation in the LAMP2 gene

Purpose: To describe the phenotype and genotype in a young woman with Danon disease. Methods: The patient underwent an ophthalmic examination including best corrected visual acuity (BCVA), fundus photography and fundus autofluorescence (FAF), full-field electroretinography (full-field ERG), multifocal ERG, optical coherence tomography (OCT) and SAP-Humphrey 30-2 at the ages of 20 and 25. Electrooculography, fluorescein angiography (FA), indocyanine angiography and OCT angiography were performed only once. Genetic testing using a Next-Generation Sequencing panel and immunohistochemical analysis of LAMP2 protein expression were performed in the patient's explanted heart, and the patient's cardiologic and ophthalmologic records were retrospectively reviewed. Results: A de novo, novel, mosaic mutation, c.135dupA; p.(Trp46Metfs*10) was identified in exon 2 of the LAMP2 gene. Immunohistochemical investigation of the myocardium in the explanted heart revealed pronounced deficiency of LAMP2 protein in cardiomyocytes. The color photographs, FAF images and FA revealed more extensive peripheral pigmentary retinal dystrophy (PPRD) at the 5-year follow-up examination. No changes were observed in BCVA, OCT, SAP-Humphrey 30-2 or multifocal ERG findings at follow-up. Full-field ERG showed an asymmetric interocular reduction in ERG response at follow-up: the b-wave amplitude of the rod response had decreased by 29% in the right eye, but by only 6 % in the left eye. The a-wave amplitude of single-flash response had decreased by 9 % in the left eye, while it had increased by 3% in the right eye. Conclusions: Although PPRD progressed slowly, it was an important clue in the diagnosis of the life-threatening condition of Danon disease.

Cardiomyopathy in Children: Classification and Diagnosis: A Scientific Statement From the American Heart Association

In this scientific statement from the American Heart Association, experts in the field of cardiomyopathy (heart muscle disease) in children address 2 issues: the most current understanding of the causes of cardiomyopathy in children and the optimal approaches to diagnosis cardiomyopathy in children. Cardiomyopathies result in some of the worst pediatric cardiology outcomes; nearly 40% of children who present with symptomatic cardiomyopathy undergo a heart transplantation or die within the first 2 years after diagnosis. The percentage of children with cardiomyopathy who underwent a heart transplantation has not declined over the past 10 years, and cardiomyopathy remains the leading cause of transplantation for children >1 year of age. Studies from the National Heart, Lung, and Blood Institute-funded Pediatric Cardiomyopathy Registry have shown that causes are established in very few children with cardiomyopathy, yet genetic causes are likely to be present in most. The incidence of pediatric cardiomyopathy is ≈1 per 100 000 children. This is comparable to the incidence of such childhood cancers as lymphoma, Wilms tumor, and neuroblastoma. However, the published research and scientific conferences focused on pediatric cardiomyopathy are sparcer than for those cancers. The aim of the statement is to focus on the diagnosis and classification of cardiomyopathy. We anticipate that this report will help shape the future research priorities in this set of diseases to achieve earlier diagnosis, improved clinical outcomes, and better quality of life for these children and their families.

A Nationwide Survey on Danon Disease in Japan

Danon disease, an X-linked dominant cardioskeletal myopathy, is caused by primary deficiency of lysosome-associated membrane protein-2 (LAMP-2). To clarify the clinicopathological features and management, we performed the first nationwide, questionnaire-based survey on Danon disease in Japan. A total of 39 patients (17 males, 22 females) from 20 families were identified in the analysis. All patients had cardiomyopathy. Of the 21 patients who died, 20 (95%) died of cardiac failure or sudden cardiac arrest. Most patients had hypertrophic cardiomyopathy. Wolf–Parkinson–White syndrome was present at a comparatively high incidence (54% in males, 22% in females). Only one female patient received a heart transplant, which is the most effective therapy. Histopathologically, all male patients showed autophagic vacuoles with sarcolemmal features in muscle. Half of the probands showed de novo mutations. Male patients showed completely absent LAMP-2 expression in muscle. In contrast, female patients showed decreased LAMP-2 expression, which is suggested to reflect LAMP-2 haploinsufficiency due to a heterozygous null mutation. In conclusion, Danon disease is an extremely rare muscular disorder in Japan. Cardiomyopathy is the most significant prognostic factor and the main cause of death. Our findings suggest that the present survey can extend our understanding of the clinical features of this rare disease.

Variable X-chromosome inactivation and enlargement of pericentral glutamine synthetase zones in the liver of heterozygous females with OTC deficiency

Ornithine transcarbamylase (OTC) deficiency is an X-linked disorder that causes recurrent and life-threatening episodes of hyperammonemia. The clinical picture in heterozygous females is highly diverse and derives from the genotype and the degree of inactivation of the mutated X chromosome in hepatocytes. Here, we describe molecular genetic, biochemical, and histopathological findings in the livers explanted from two female patients with late-onset OTC deficiency. Analysis of X-inactivation ratios by DNA methylation-based assays showed remarkable intra-organ variation ranging from 46:54 to 82:18 (average 70:30, n = 37), in favor of the active X chromosome carrying the mutation c.583G>C (p.G195R), in the first patient and from 75:25 to 90:10 (average 82:18, n = 20) in favor of the active X chromosome carrying the splicing mutation c.663+1G>A in the second patient. The X-inactivation ratios in liver samples correlated highly with the proportions of OTC-positive hepatocytes calculated from high-resolution image analyses of the immunohistochemically detected OTC in frozen sections that was performed on total area > 5 cm². X-inactivation ratios in blood in both female patients corresponded to the lower limit of the liver values. Our data indicate that the proportion of about 20-30% of hepatocytes expressing the functional OTC protein is not sufficient to maintain metabolic stability. X-inactivation ratios assessed in liver biopsies taken from heterozygous females with X-linked disorders should not be considered representative of the whole liver.

Transient Ischemic Attack and Ischemic Stroke in Danon Disease with Formation of Left Ventricular Apical Thrombus despite Normal Systolic Function

Danon disease is a rare X-linked dominant skeletal and cardiac muscle disorder presenting with hypertrophic cardiomyopathy, Wolf-Parkinson-White syndrome, skeletal myopathy, and mild intellectual disability. Early morbidity and mortality due to heart failure or sudden death are known in Danon disease, more in males than in females. Here, we present a 17-year-old female adolescent with Danon disease and severe concentric hypertrophy with normal left ventricular (LV) systolic function, who has been complaining of intermittent headache and weakness for about 3 years, initially diagnosed with hemiplegic migraine. Subsequently, her neurological manifestation progressed to transient ischemic attack (TIA) and eventually to ischemic stroke confirmed by CT scan with 1-day history of expressive aphasia followed by persistent left side weakness and numbness. Detailed echocardiogram for the first time revealed a small LV apical thrombus with unchanged severe biventricular hypertrophy and normal systolic function. This unexpected LV apical thrombus may be associated with a wide spectrum of neurological deficits ranging from TIA to ischemic stroke in Danon disease. Possibility of cerebral ischemic events should be suspected in Danon disease when presenting with neurological deficits even with normal systolic function. Careful assessment for LV apical thrombus is warranted in such cases.

Female Fabry disease patients and X-chromosome inactivation

Fabry disease is an X-linked inherited lysosomal storage disorder caused by mutations in the gene encoding α-galactosidase A (GLA). Once it was thought to affect only hemizygous males. Over the last fifteen years, research has shown that most females carrying mutated allele also develop symptoms, demonstrating a wide range of disease severity, from a virtually asymptomatic to more classical profile, with cardiac, renal, and cerebrovascular manifestations. This variable expression in females is thought to be influenced by the process of X-chromosome inactivation (XCI). The aim of this study was to assess severity of the clinical phenotype, to analyze XCI patterns, and to estimate their effect on disease manifestation in twelve female Fabry disease patients from five unrelated Polish families. Our analyses revealed that patients presented with the broad range of disease expression - from mild to severe, and their clinical involvement did not correlate with XCI profiles. Female carriers of the mutation in the GLA gene with the random XCI may present with the wide range of disease signs and symptoms. Thus, XCI is not a main factor in the phenotype variability of Fabry disease manifestation in heterozygous females.

Ischemic stroke due to hypoperfusion in a patient with a previously unrecognized Danon disease

Danon disease, an X-linked multisystemic disorder, is due to deficiency of Lysosome-Associated Membrane Protein 2 (LAMP2). It is usually characterized by hypertrophic cardiomyopathy, mental retardation and skeletal myopathy, sometimes also with atypical features. A 20-year-old man with cognitive impairment was admitted to the Emergency Room because of a sudden chest pain. ECG showed Wolff-Parkinson-White syndrome; echocardiography revealed hypertrophic cardiomyopathy, and, shortly after, he experienced a cardiac arrest followed by an occipital ischemic stroke. On neurological examination, he complained of visual loss, and diffuse muscle wasting and weakness were also unexpectedly noted. Electromyography evidenced a myopathic pattern and a peripheral neuropathy. A muscle biopsy disclosed vacuolar myopathy with glycogen storage; immunohistochemical studies demonstrated a LAMP-2 deficiency. LAMP2 molecular analysis identified a "de novo" mutation (p. Q353X). This patient with a neglected Danon disease, experienced an unusual complication as a stroke due to cerebral hypoperfusion after cardiac arrest caused by WPW syndrome.

Identification of LAMP2 Mutations in Early-Onset Danon Disease With Hypertrophic Cardiomyopathy by Targeted Next-Generation Sequencing

Danon disease is an X-linked disorder with the clinical triad of cardiomyopathy, skeletal myopathy, and mental retardation. Early diagnosis of this disease remains a challenge, especially in the pediatric population. In this study, we developed a targeted panel-based next generation sequencing pipeline to identify mutations by sequencing of selected candidate genes in 136 pediatric patients with either hypertrophic cardiomyopathy (HC) or idiopathic dilated cardiomyopathy (IDC). This led to the identification of lysosome-associated membrane protein 2 (LAMP2) mutations in 4 of the 64 (6%) probands with HC, including 3 novel nonsense mutations (p.Q240X, p.S250X, and p.G22X). No LAMP2 mutation was detected in the other 72 probands with IDC. All 4 probands and one additional affected family member (2 men and 3 women) had an early-onset age and presented either HC alone or combined with Wolff-Parkinson-White syndrome and skeletal myopathy. Immunofluorescence staining and Western blot analysis revealed absent LAMP2 expression in both cardiac and skeletal muscle samples of the first proband and severely decreased LAMP2 expression in the skeletal muscle samples of the second proband. In conclusion, cardiomyopathy in the patients with Danon disease may occur during early childhood and tend to be HC rather than IDC in both affected men and women. Therefore, Danon disease should be considered as one of the leading causes of unexplained ventricular hypertrophy in pediatric patients. The inclusion of LAMP2 gene in cardiomyopathy genetic screening panels may contribute to early diagnosis of Danon disease.

A novel LAMP2 mutation associated with severe cardiac hypertrophy and microvascular remodeling in a female with Danon disease: a case report and literature review

BACKGROUND

Danon disease (DD) is a rare disorder characterized by cardiomyopathy, intellectual disability, and proximal myopathy. It is caused by mutations in the LAMP2 gene on X chromosome. Female patients most often present with late-onset cardiomyopathy and slow disease progression, but early-onset cases with unfavorable prognosis have been reported.

CASE REPORT

We describe the clinical, pathological, and molecular features of a novel LAMP2 c.453delT mutation in a female patient with severe hypertrophic cardiomyopathy, Wolff Parkinson White (WPW) syndrome and rapid progression to heart failure, requiring heart transplant. Immunohistochemical analysis of LAMP2 in the explanted heart revealed a mosaic pattern of distribution, with discrete clusters of either stained or unstained cardiac myocytes, the latter being more frequent in the septum. These findings paralleled X chromosome inactivation within the myocardium. Interestingly, multiple foci of microscarring were found on histology in the Left Ventricle (LV) free wall and septum, in a close spatial relationship with remodeling and severe stenosis of intramural coronary arterioles.

CONCLUSIONS

Our findings suggest that several features may contribute to the early and severe cardiac phenotype in female DD patients. The type of mutation may account for the early disease onset, while both the inhomogeneous distribution of LAMP2 loss and the presence of microvascular remodeling may be determinant in the rapid progression to heart failure.

Danon disease - dysregulation of autophagy in a multisystem disorder with cardiomyopathy

Danon disease is a rare, severe X-linked form of cardiomyopathy caused by deficiency of lysosome-associated membrane protein 2 (LAMP-2). Other clinical manifestations include skeletal myopathy, cognitive defects and visual problems. Although individuals with Danon disease have been clinically described since the early 1980s, the underlying molecular mechanisms involved in pathological progression remain poorly understood. LAMP-2 is known to be involved in autophagy, and a characteristic accumulation of autophagic vacuoles in the affected tissues further supports the idea that autophagy is disrupted in this disease. The LAMP2 gene is alternatively spliced to form three splice isoforms, which are thought to play different autophagy-related cellular roles. This Commentary explores findings from genetic, histological, functional and tissue expression studies that suggest that the specific loss of the LAMP-2B isoform, which is likely to be involved in macroautophagy, plays a crucial role in causing the Danon phenotype. We also compare findings from mouse and cellular models, which have allowed for further molecular characterization but have also shown phenotypic differences that warrant attention. Overall, there is a need to better functionally characterize the LAMP-2B isoform in order to rationally explore more effective therapeutic options for individuals with Danon disease.